1. Technical Field
The present invention relates to a DC/DC converter and a display device including the same and, more particularly, to a DC/DC converter, in the case where the DC/DC converter is constructed by a switching circuit, in which deterioration in efficiency of the DC/DC converter is minimized and EMI is also reduced and a display device including the same.
2. Background Art
In recent years, as the panel of a thin display device becomes larger and comes to have higher resolution (higher definition), the transmission frequency of a video signal in the device is increasing. As the size becomes larger and the resolution becomes higher, unnecessary radiation (EMI: Electromagnetic Interference) generated from the display device also becomes larger.
As radiant sources of EMI, there are noises of an IC for outputting a clock signal on a signal processing board (printed board) on which circuit parts for driving a display device are mounted and a power supply circuit necessary to drive the IC.
Particularly, the case where the power supply circuit is constructed by a switching power supply will be considered. When a switching method is used at the time of converting voltage in the power supply circuit (DC/DC converter circuit), designing can be made with relatively high efficiency. Consequently, the switching power supply circuit is a circuit system which is often used.
It is, however, known that in the circuit configuration of the switching power supply circuit, necessary voltage is generated by switching, so that voltage fluctuation (ringing) having a certain frequency component occurs at the moment of on/off switching, and the voltage fluctuation is the cause of occurrence of unnecessary noise.
There is also the possibility that the unnecessary noise component of the ringing is largely influenced by resonance frequency of the printed board itself and the unnecessary noise component is increased.
With respect to EMI, the limit value on the permissible level of noise is determined for each frequency, and all of noise levels in a specified frequency band have to be set to the limit value or less. Obviously, the lower the noise level, the better. Therefore, as will be described later, a technique of reducing EMI even in the case of using a switching power supply is known.
The conventional technique of patent literature 1 relates to a drive circuit for a semiconductor optical amplifier gate switch, a semiconductor optical amplifier gate switch device, and an optical switching device and, particularly, is directed to reduce a ringing component by inserting a diode at a post stage of an inductor (refer to FIG. 14).
However, since the ringing frequency itself which worsens EMI cannot be changed by the diode, there is a problem such that a sufficient EMI reduction effect cannot be obtained.
The conventional technique of patent literature 2 relates to a switching power supply device and, particularly, is directed to reduce ringing at the time of switching by providing snubber circuits at both ends of an inductor (transformer) (refer to FIG. 15).
In the configuration of the conventional technique, however, since the snubber circuits always operate at the time of switching, the efficiency of the power supply is worsened.
In addition, the ringing frequency component which worsens EMI cannot be sufficiently eliminated by the snubber circuit, and there is consequently a problem such that a sufficient EMI reduction effect cannot be obtained.
The conventional technique of patent literature 3 relates to a self-excited DC/DC converter and, more particularly, provides a self-excited DC/DC converter which suppresses ringing by extending an off period of a switching element and can suppress increase in the frequency at the time of low load and contribute to improvement in efficiency at the time of low load (refer to FIG. 16).
However, the ringing mentioned in the conventional technique is ringing which is about a few times as high as oscillation frequency generated at the time of low load but is not ringing which is a few hundreds times to a few thousands times as large as that which occurs at the on/off timings of the oscillation frequency. Since a ringing frequency component in the band of tens or hundreds MHz which exert largest influence on EMI cannot be eliminated, there is, consequently, a problem that the EMI reduction effect cannot be obtained.
The conventional technique of patent literature 4 relates to a drive circuit of a voltage driving element and, more particularly, to a technique of providing a snubber circuit between a load such as a three-phase inverter and an input Vin, sending a feedback (F/B) to the input, and making gate opening/closing at the time of switching dull, thereby reducing ringing (refer to FIG. 17).
In the conventional technique, however, two protection diodes and three snubber resistors are necessary in addition to the present invention.
In the configuration of the conventional technique, in the case where the snubber circuit operates at the time of switching, a loss due to the snubber resistor occurs, so that the efficiency deterioration is caused.
Further, due to the method of eliminating ringing by feeding back (F/B) the voltage fluctuation in the load to the input to make the opening/closing timing of the gate of the switching FET dull, the efficiency deterioration becomes larger by the gate opening/closing timing.
In addition, by sending an F/B of the load to the input line, there is the possibility that noise having a ringing component is diffused into the input line.
The conventional technique of the patent literature 5 relates to an adaptive ringing suppressing device. In the configuration of the conventional technique, ringing is reduced by using a low-pass filter circuit including a delay circuit (refer to FIG. 18). By using the low-pass filter in a video signal processing circuit for a video signal or the like including a ringing component, the ringing component is eliminated.
However, the conventional technique does not eliminate a ringing component in a place where ringing occurs but an output from which the ringing component is eliminated as a result of passing through circuits is obtained.
That is, there is a part of an output of a single delay circuit, which is transmitted to the next stage while including the ringing component. The ringing component cannot be fundamentally eliminated. Even when the method of the conventional technique is applied, EMI cannot be reduced.